IO and STM

[Lemmih]

On 4/13/05, Robert van Herk <rherk at cs.uu.nl> wrote:
> Hi all,
> 
> I have a question about IO in a STM transaction.
> 
> I want to make an atomic algorithm that does the following:
> 
> 1. read an IORef
> 2. write some changes to the value you get from 1
> 3. save the IORef again
> 
> Currently, I did this, roughly like this:
> 
> doSomething :: IORef [s] -> s -> IO ()
> doSomething ref s =
>   atomically (
>     do { ss <- unsafeIOToSTM (readIORef s);
>          do something with ss;
>          unsafeIOToSTM (writeIORef s ss);
>          return ()
>        }
>   )
> 
> Since I read that STM uses a revertable log, I was wondering what
> happens with the IO actions, when a rerun on the atomic part is
> performed... I guess it will just redo them? Is this the unsafe-part of
> unsafeIOToSTM, or are there more caveats?

You probably wanna use TVars instead of IORefs.
I would suggest reading the STM paper at
research.microsoft.com/~simonpj/papers/stm/stm.ps, if you haven't
already.

-- 
Friendly,
  Lemmih